1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly to an active matrix type liquid crystal device using a thin film semiconductor device and a method for manufacturing the same.
2. Description of Related Art
An active matrix type liquid crystal device is in a trend of high fineness and multi-tonality. In order to achieve it, it is essential to form an additional capacitance for each pixel. The additional capacitor is connected to a TFT in parallel with a capacitor of a liquid crystal layer to function to prevent a drop of a voltage to be applied to the liquid crystal layer due to a leakage current of the TFT. A typical construction thereof is disclosed in JP-A-1-267618.
When the prior art additional capacitor is applied to a construction disclosed in JP-A-2-149824 to prevent a short circuit between electrodes, an insulation film of the additional capacitor is a single-layer insulation film and hence it is difficult to maintain a sufficiently high yield in the insulation. Thus, it is difficult to attain a construction which has a highly reliable additional capacitor and reduces the problem of short circuit, merely by a combination of the prior art techniques.
Further, since it is necessary to maintain a certain distance between a pixel electrode and a video signal wiring in order to prevent short circuit between the pixel electrode and the video signal wiring, a space to be occupied by the wiring increases and a sufficient area for the pixel electrodes is not secured. Thus, as an aperture ratio decreases, a surface brightness of the display device is lowered. In order to compensate for it, it is necessary to increase a brightness of a back light, as a result, a cost increases. This problem is more serious as the display device becomes fixed. In order to suppress the reduction of the aperture ratio, it has been suggested to reduce a wiring width of a non-multi-wired portion in order to reduce the area occupied by the wiring, but a problem of breakage of the wiring increases as the wiring width is reduced.
An oxide film of an indium-tin alloy (hereinafter referred to as an ITO film) has been most commonly used as the fixed electrode, and a silicon nitride film (hereinafter referred to as an SiN film) formed by a plasma chemical vapor phase deposition method (hereinafter referred to as a PCVD method) has been most commonly used as first and second insulation films which are gate insulation films. It has been known that when the SiN film is deposited on the ITO film by the PCVD method, the surface of the ITO film is reduced by a reducing atmosphere in the film formation process so that the transparency of the ITO film is lost. In order to prevent it, it has been proposed in JP-A-59-9962 to protect the ITO film by an insulation film such as an SiO.sub.2 film prior to the formation of the SiN film. However, the method increases the number of steps of the process and hence the cost.
Further, in the prior art, image sticking is apt to happen because the two insulation layers, the protective insulation layer and the gate insulation layer, are deposited on the pixel electrodes.